The invention relates to a signal processing device having an analogue input, a shift register or data memory, and A/D converter, and a processor producing a signal course to be displayed, as preferably used as a digital memory oscillograph, among other uses.
A digital memory oscillograph is used to display waveforms. An analog signal is digitized. The digital data are stored for the ensuing display of the analog waveform or for a calculation by means of a digital processor (.mu.P).
Digital memory oscillographs of this type are becoming increasingly important because with them unique features, such as pre- and post-triggering, an arbitrarily long storage time and various kinds of signal processing, that are unattainable with conventional oscillographs that operate in real time, become possible. These features, in combination with advanced digital technology, make it possible to increase the scanning speed and reduce the memory capacity.
In this connections, provisions are also known that use integrated circuits and large-scale-integration (LSI) technology and have analog memories for high speeds, for instance CCD (charge coupled device) circuits, as analog shift registers. German patent disclosure document DE-OS 33 38 381 discloses an improved memory oscillograph that includes this kind of analog shift register. The disadvantage in the use of CCD circuits, however, is that these memories are incapable of functioning independently of the time base selected. The above-mentioned document accordingly describes an oscillograph that functions in a plurality of different operating modes in accordance with the setting of the time base. This kind of provision is unnecessarily expensive, however, and for the most part it cancels out the advantages generally attainable with digital processing.
The problems presented here can be described as follows:
Digital oscillographs acquire electrical signals by quantizing scanned values, which are recorded at separate time intervals and subsequently are entered in quantized form into a memory for later display as a regenerated wave form. Initially the apparent band width of digital oscillographs was limited, because all the scanned values that belong to one waveform had to be recorded in succession, each in a single period of the signal having the scanning clock frequency. The reason for this was the asynchronous relationship between the triggering event and the scanning clock rate. In the field of digital oscillographs, this mode of operation is known as acquisition once per clocking.
In a mode of operation with signal acquistion similar to the chronologically equivalent random sampling acquisition, the apparent band width of signal acquisition was expanded considerably. In this mode, locations in the various periods of a repeating signal are detected, and a single chronologically equivalent period of the signal is reestablished from these locations, although the waveform scanning values over the course of many periods can be recorded separately from one another. One problem associated with this kind of chronologically equivalent waveform regeneration is that it takes a relatively long time to detect all the relevant scanning values, displaying the various data points.
Another problem is that the trigger point, which is the same in all successive periods of the signal, and the scanning cycle, which has a predetermined fixed frequency, are not matched to one another. As a result, the data points shown exhibit a horizontal fluttering relative to one another. This problem has been addressed in U. S. Pat. No. 4,251,754 of Luis J. Navarro and Thomas P. Dagostino. It is explained in this patent that the fluttering caused by the indefiniteness of scanning can be corrected by measuring the time interval between a trigger recognition event (which occurs when the signal potential passes through a selectable threshold value) and the next subsequent scanning clock pulse and by using the measured value to generate an offset current in the horizontal displaying system; as a result of this correction, each frame of the display is shifted horizontally. In this process, every scanning value represented is put into its precise chronological location. This solution of the fluttering problem is not, however, suitable for waveform processing with a computer or the like that takes place in the meantime, because the correction takes place only in the displaying system.
Another aspect in chronologically equivalent waveform regeneration is the influence of the lower Nyquist threshold on the scanning frequency of the periodic scanning. If the scanning frequency is less than twice the frequency of the input signal, then data is lost because there are too few scans. Under some circumstances, the result may be a waveform displayed in distorted form. It was accordingly desirable both to obtain numerous scanning values for each trigger recognition and to provide for the precise chronologically equivalent location of each scanned data point in the acquisition process.
In this connection, from German patent disclosure document DE-OS 33 27 139, a method for detecting a chronologically equivalent waveform during a great number of periods of a repeating input signal is known, in which one or more scanning values are detected from each period of the many periods at separate intervals of one scanning clock rate; the chronological difference between a fixed point in each period and the next successive clock pulse is measured; from the measured chronological difference a memory address is determined for each scanning value; and the scanning values are stored in the memory at the address calculated.
This method is again very complicated and with it, a signal course cannot be displayed on the screen until some time after its appearance at the input of the oscillograph, because the above-described signal processing takes a certain amount of time. This is particularly annoying because it involves events that already elapse relatively slowly.